1. Field of the Invention
This invention relates to a radiation image read-out method for reading out a radiation image stored on a stimulable phosphor sheet by exposing the stimulable phosphor sheet to stimulating rays which cause it to emit light in proportion to the stored radiation energy, and photoelectrically detecting the emitted light, and an apparatus for carrying out the method. This invention particularly relates to a radiation image read-out method wherein the detection level for the light emitted by the stimulable phosphor sheet is automatically adjusted to an optimal level in accordance with image recording conditions.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to electric image signals, and the radiation image of the object is reproduced as a visible image by use of the image signals on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range of radiation exposure and further in that the electric signals used for reproducing the visible image can be freely processed to improve the image quality for viewing, particularly for diagnostic purposes. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor sheet varies over a wide range in proportion to the amount of said stored energy, it is possible to obtain an image having a desirable density regardless of the amount of exposure of the stimulable phosphor sheet to the radiation, by reading out the emitted light with an appropriate read-out gain and converting it into electric signals to reproduce a visible image on a recording medium or a display device. The electric signals may further be subjected to appropriate image processing (signal processing) such as gradation processing or frequency response enhancement processing to obtain a radiation image suitable for viewing, particularly for diagnostic purposes. This is very advantageous in practical use.
In order to obtain a radiation image having a high image quality or a high diagnostic efficiency and accuracy by carrying out image processing such as gradation processing, it is necessary to ascertain such image input conditions of the radiation image stored on the stimulable phosphor sheet as, for example, the level of radiation dose used for image recording, or the image input pattern which is determined by the portion of the object (e.g., the chest or the abdomen of the human body) or the image recording method used, such as plain image recording or contrasted image recording, and then to adjust the image processing conditions to appropriate values based on the ascertained image input conditions or the image input pattern. The image input conditions and the image input pattern will hereinafter be simply referred to as the image input information when they are referred to generically.
Ascertaining of the image input information on the stimulable phosphor sheet may be carried out prior to the visible image reproduction by use of the method disclosed in U.S. Pat. No. 4,527,060. In the disclosed method, a read-out operation for ascertaining the image input information of a radiation image stored on a stimulable phosphor sheet (hereinafter referred to as the preliminary read-out) is carried out in advance by use of stimulating rays having stimulation energy of a level lower than the level of the stimulation energy of stimulating rays used in a read-out operation for obtaining a visible image for viewing, particularly for diagnostic purposes (hereinafter referred to as the final read-out). The image processing conditions are adjusted on the basis of the image input information obtained by the preliminary read-out.
However, in order to carry out the preliminary read-out in the manner as mentioned above, a read-out system for the preliminary read-out is necessary besides the read-out system for the final read-out. Or, in the case where a single read-out system is utilized for both the preliminary read-out and the final read-out, a complicated sheet conveyance mechanism is necessary for conveying the stimulable phosphor sheet reversely to the direction of conveyance for sub-scanning with stimulating rays after the preliminary read-out is finished, and then again conveying the stimulable phosphor sheet for the sub-scanning. Also, in the case where the preliminary read-out is carried out in the manner mentioned above, the time taken for read-out processing is prolonged.
Accordingly, the applicant proposed in, for example, Japanese Patent Application No. 61(1986)-174023 a method wherein the preliminary read-out is omitted, the image input information on the stimulable phosphor sheet is ascertained from image signals obtained by the final read-out, and image processing conditions are adjusted to optimal values based on the ascertained image input information.
However, in the case where the preliminary read-out is omitted, it becomes difficult to adjust the read-out conditions for the final read-out to appropriate values. Therefore, there is a risk of the light detection range of a light detection means such as a photomultiplier for the final read-out being adjusted to an incorrect range unsuitable for the image input information on the stimulable phosphor sheet. This problem will hereinbelow be described in detail.
In general, in the case where a radiation image of an object such as the human body is read out, it is sufficient for the read-out range of the light detection means in the final read-out to be two orders of ten in terms of relative sensitivity. However, the intensity range of light emitted by a stimulable phosphor sheet changes markedly in accordance with the radiation image recording conditions, for example, the image recording portion of the object and the image recording method such as normal image recording or enlarged image recording. For example, in mammography or the like, the level of light emitted by the stimulable phosphor sheet becomes markedly higher than in a radiation image of a general object portion. In order to accurately detect all types of radiation images recorded under different image recording conditions, the read-out range must be increased by at least one order of ten to each of the upper and lower sides of the aforesaid read-out range, i.e., a read-out range of at least four orders of ten is necessary in terms of relative sensitivity.
In the preliminary read-out, image read-out is generally carried out coarsely, and the time taken for the image read-out is short. Therefore, a large current can be applied to the light detection means such as the photomultiplier, and frequency response characteristics of a logarithmic amplifier for amplifying the output of the light detection means may not be so high. Accordingly, in the preliminary read-out, the read-out range can be adjusted to at least five orders of ten in terms of relative sensitivity. However, in the final read-out, the time taken for image read-out is long, and therefore it is not always possible to apply a large current to the photomultiplier or the like. As a result, the read-out range on the high light emission region side is limited. On the other hand, it is necessary to keep the frequency response characteristics high in order to obtain image signals of high quality, and the read-out range on the low light emission region side is limited thereby. Consequently, the read-out range for the final read-out is limited to four orders of ten at the most in terms of relative sensitivity.
Therefore, in the case where the read-out range for the final read-out is fixed to four orders of ten in terms of relative sensitivity, the intensity range of the light emitted by the stimulable phosphor sheet goes beyond the read-out range and it becomes impossible to detect all the light emitted by the stimulable phosphor sheet when the aforesaid image recording conditions change and the light emission intensity range deviates markedly toward the low light emission region side or toward the high light emission region side.